(a) Field of the Invention
The present invention relates to a thermoplastic polyimide foam which is excellent in resistance to heat, flame and abrasion, and also has good fabrication quality.
(b) Description of the Related Art
Foams which have conventionally been developed by use of macromolecular materials are roughly divided into the following two classes.
(1) Foams prepared from thermoplastic resins, for example, polystyrene, polyvinyl chloride, polypropylene and polyethylene.
(2) Foams prepared from thermosetting resins, for example, polyurethane resin, phenol resin, urea resin, melamine resin and imide resin.
The former foams are excellent in abrasion resistance and fabrication quality and hence are broadly applied to automobiles, buildings or electronic and electric fields by utilizing substantial characteristics such as heat insulation property and light-weight property. However, the foams are insufficient in thermal resistance and particularly in flame resistance. For example, shape of the foams are drastically deformed at high temperatures such as above 150.degree. C. and performance of foam is eliminated. When the foams are brought into contact with a flame, combustion occurs from the surface of the foams. Even though flame resistance is provided by addition of a flame retarder, the foams are immediately fused and drip down at the portion contacting the flame. Thus adjacent portions of the foams are successively melted and finally most of the foam disappears. As a result of such disadvantage, the foams cannot be applied to fields such as buildings and aircraft parts where flame resistance is required.
The latter foams which are prepared from thermosetting resin, for example, polyurethane resin foam have also the above disadvantage against the flames. On the other hand, the foams prepared from phenol resin, urea resin and melamine resin have excellent flame resistance and do not melt upon contact with flames. However, these foams are poor in abrasion resistance, liable to crack and are impossible to fabricate. These demerits have restricted fields of application.
Polyimide resin foams have also been developed. A representative polyimide foam (A) is based upon a prepolymer of International Harvester Co. which is prepared by esterifying an aromatic carboxylic acid dianhydride with an alcohol and adding a diamine. Another typical polyimide foam (B) is based upon a polyetherimide (ULTEM 1000.TM. etc., products of General Electric Co.) which is prepared from a tetracarboxylic acid dianhydride having ether bonds in the molecule.
The foam (A) has an outstanding thermal resistance and does not deform particularly at a temperature of 200.degree. C. Flame resistance is also excellent because the foam (A) does not melt in contact with the flame. On the other hand, abrasion resistance of the foam (A) is insufficient and fabrication is impossible. Once the foam (A) is made, it cannot be used for various applications by conducting fabrication and hence restriction has been imposed upon the foam (A).
The foam (B) has been improved in flame resistance, abrasion resistance and fabrication quality. However, thermal resistance is still unsatisfactory.